In automobile assembly plants and other applications, large conveyor systems are commonly used to convey articles such as partially completed automobile bodies between work stations arranged along the conveyor path. One type of conveyor system that has been popular in this type of application is known as a power and free conveyor. A power and free conveyor includes a chain which is driven continuously along a track. The automobile bodies or other articles are conveyed on carriers which are mounted on wheeled trolleys riding on another track. The chain has spaced apart drive dogs that are detachably engaged with retractable dogs on the trolleys so that each trolley which has its retractable trolley dog engaged by a chain dog is driven along the conveyor path.
Power and free conveyors can be overhead systems where the load is suspended from the carrier. Alternatively, inverted power and free conveyors have the load on top of the carrier with the chain running beneath the trolleys and the chain dogs projecting upwardly.
Normally, a number of work stations are located along the conveyor where the carriers are stopped so that various types of operations can be performed on the work. Stop mechanisms have been developed for detaching the trolley dog from the chain dog in order to stop the carrier at the location of each work station. For example, retractable blades and cams can be installed at each work station and extend into the path of an approaching trolley. A pivot arm carrying the trolley dog is engaged by the stop device and pivoted in a direction to detach the trolley dog from the chain dog, thus stopping the trolley while the chain continues to move and convey other trolleys along the conveyor path.
Accumulation of carriers occurs in this type of conveyor system. If a carrier is stopped and the succeeding carrier approaches it, the incoming carrier must be stopped before it bangs into the stopped carrier and causes damage to the equipment and possibly to personnel in the vicinity. Accumulation devices of various types have been used to stop carriers which approach preceding carriers that are stationary. Usually, the trolley on the trailing end of each carrier is equipped with a cam surface which is engaged by the pivot arm of the trolley on the leading end of the succeeding carrier. When the pivot arm comes into contact with the cam surface, the trolley arm is pivoted to a retracted position wherein the trolley dog is released from the drive dog on the chain.
Accumulator mechanisms of this type are characterized by cost and complexity and have been plagued by other problems, particularly in the area of reliability. In order to function properly, the trailing cam surface must be located properly to intercept the incoming trolley arm. There is little tolerance for error, so if either part is bent up or down or sidewardly, or if the parts are misaligned slightly for some other reason, the pivot arm may not be retracted far enough to fully release the trolley dog from the chain dog, and serious adverse consequences can result. Conversely, the pivot arm may be subjected to over travel which can bend or break the arm or associated parts of the trolley. In either case, costly repairs must be made to the conveyor equipment, and it may be necessary to take the system out of production for an extended time while it is being repaired.